Ultrasonic sensors having a transmitter disposed on one side of a web and a receiver disposed on the other side of the web for locating the position of an edge of the web therebetween, are known in the art. Such ultrasonic sensors are mounted perpendicular to the web direction of travel and suffer from problems due to the reflection of sound between the transmitter, the web, and the receiver. Noise is thereby introduced into the signal produced by the receiver, and reduces the accuracy of the prior art ultrasonic sensors.
Previously, people in the art attempted to solve this problem by changing the angle of the ultrasonic sensor with respect to the web (away from the perpendicular orientation discussed above), as well as designing sensor housing to maintain non-parallelism between transmit and receive sides, so that the sound reflections would be reflected away from the receiver. Although the prior art method reduced the problem of sound reflection somewhat, it is less desirable to have the prior art ultrasonic sensor mounted angularly with respect to the direction of web travel. In addition, this angular mounting limits the effective sensing gap and allows less web plane change than that of perpendicular mounting.
Thus, there is a need for an ultrasonic sensor which does not suffer from the aforementioned problems caused by the reflection of the sound waves as described hereinabove. It is to such an improved ultrasonic sensor that the present invention is directed.